Clinical and preclinical data suggest that estrogen depletion and replacement impact cognitive function, and experimental data suggest that this may involve synaptic changes in hippocampus and prefrontal cortex. However, aging also affects cognition and related circuits, thus, we need to understand how neural and endocrine senescence interact if we are to understand the neurobiology of menopause and design appropriate hormone replacement strategies. We will continue to investigate both rat and non-human primate models of estrogen depletion and replacement to elucidate the synaptic basis of estrogen-induced cognitive enhancement, and age-related differences in response to estrogen. Expansion over the last funding period will include additional molecular targets for electron microscopy, an increased focus on prefrontal cortex, and new hormone replacement protocols that have high clinical relevance. Specific Aim 1 will determine how long-term cyclical exposure to estrogen affects spine number, pyramidal cell morphology, and the molecular profile of excitatory synapses in hippocampus and prefrontal cortex of young and aged rhesus monkeys, with particular focus on NMDA receptors, estrogen receptors, and related signaling molecules. We will then determine which of these indices are most directly related to the cognitive enhancement that follows from this treatment. Specific Aim 2 will use the rat model to elucidate the molecular basis of the age-related loss of synaptic resiliency in response to estrogen, as well as the effects of progesterone on estrogen-induced plasticity. We hypothesize that a loss of synaptic resilience in the aged females makes them more dependent on estrogen for optimal performance. Aim 3 will employ multiple hormone replacement regimens in young ovariectomized monkeys to investigate the neurobiological effects of chronic vs., cyclical estrogen, as well as the impact of continuous or cyclical progesterone on estrogen-induced plasticity. We hypothesize that chronic and cyclical exposure to progesterone will differentially modulate the effects of estrogen. Specific Aim 4 will use a similar comprehensive set of hormone replacement regimens in aged behaviorally characterized monkeys to determine which regimens are most successful at restoring youthful synaptic, cellular, and behavioral profiles. Such data will inform the clinical community on which hormone replacement regimens are appropriate for the aging brain.